Bordetella avium infection of birds serves as an animal model for B. perutssis infection of man since both fowl and humans with bordetellosis exhibit similar clinical symptoms and histopathology. Cosmid libraries were prepared with B. avium DNA and six recombinant E. coli clones were identified that reacted with antibody against B. avium outer membrane proteins and convalescent turkey sera. The proteins encoded by the six recombinant cosmids had molecular masses of 21 kDa, 38 kDa, 40 kDa, 43 kDa, 48 kDa, and 56 kDa. The gene for the 21 kDa outer membrane protein was localized by Tn5seq1 mutagenesis and the nucleotide sequence was determined by dideoxy sequencing. DNA sequence analysis of the 21 kDa protein revealed an open reading frame of 582 bases that resulted in a predicted protein of 194 amino acids and a molecular weight of 21, 113. The 21 kDa B. avium outer membrane protein exhibited significant homology to the carboxy terminus of OmpA proteins of Shigella dysenteriae, E. coli, Salmonella typhimurium, and Neisseria gonorrhoeae outer membrane protein III. Based on this homology, the B. avium 21 kDa protein gene was designated ompA. A 6.75 kb DNA fragment encoding B. avium was subcloned into the Asd+ vector pYA292 and the construct was introduced into an avirulent delata asdm, delta cya, delta crp S. typhimurium X3987 for oral immunization of birds. Fractionation of S. typhimurium X3987 producing B. avium OmpA inciated that the protein was localized primarily in the cytoplasmic membrane and the outer membrane of S. typhimurium X3987. Southern hybridization of the B. avium ompA gene to chromosomal DNA of the other Bordetella species indicated that both B. pertussis and B. bronchiseptica contain genes which are homologous to B. avium ompA. In addition, the regulation of virulence genes in B. avium was investigated. Hybridization of B. avium chromosomal DNA with B. pertussis bvgS and bvgA regulatory genes revealed that B. avium contained a B. pertussis bvgS homology but lacked a bvgA homology.